A steel material such as a carbon steel or a low alloy steel is used for oil well steel pipes, line pipes, etc. for the production and transportation of petroleum, gas, and the like. For highly corrosive petroleum and gas wells, a carbon steel, or the like material is used while corrosion inhibitors are added to petroleum, etc., or a stainless steel material such as 13% Cr steel is used as the material itself. A sufficient service period of the oil well steel pipes and line pipes has thus been ensured.
However, use of a stainless steel for oil wells, etc., the life of which is short is overly expensive in terms of cost efficiency because the stainless steel is costly. On the other hand, there is a trend towards avoiding the use of corrosion inhibitors because of their adverse effects on the environment. Accordingly, the development of steel materials capable of ensuring the resistance to corrosion to a certain degree has been expected. In order to answer the expectations, a steel containing 0.5 to 5% of Cr is proposed in Japanese Unexamined Patent Publication (Kokai) No. 57-5846. However, the patent publication merely discloses in examples inventive steels containing from 0.5 to 2.1% of Cr.
Furthermore, when a steel containing Cr is used, the steel must have a good balance between the strength and the low temperature toughness. Although the balance therebetween is greatly influenced by the metallic structure, Japanese Unexamined Patent Publication (Kokai) No. 57-5846 defines no metallic structure of the steel material.